Ceramic honeycomb structure and honeycomb-molding die

ABSTRACT

A ceramic honeycomb structure comprising large numbers of flow paths longitudinally formed by cell walls arranged in a lattice pattern in cross section, and an outer peripheral wall formed around the flow paths; in a cross section perpendicular to the longitudinal direction, fan-shaped bulges projecting in a fan shape toward the flow paths from cell wall intersections at which the cell walls are crossing; the circumscribed circles of circular portions of the fan-shaped bulges at all cell wall intersections having a constant radius; and when the distance between the center point of the circumscribed circle and the center point of the cell wall intersection is defined as a center point distance S, a center point distance So in the outer peripheral portion of the ceramic honeycomb structure and a center point distance Sc in the center portion meeting Sc&lt;So.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2019/033692 filed Aug. 28, 2019, which claims priority to JapanesePatent Application No. 2018-161194 filed on Aug. 30, 2018.

FIELD OF THE INVENTION

The present invention relates to a ceramic honeycomb structure and ahoneycomb-molding die.

BACKGROUND OF THE INVENTION

Ceramic honeycomb structures are used for catalyst carriers and filtersin exhaust gas-cleaning apparatuses for internal engines. As shown inFIG. 15 , a ceramic honeycomb structure 60 comprises an outer peripheralwall 61, and large numbers of flow paths 63 partitioned by porous cellwalls 62 inside the outer peripheral wall 61. Such ceramic honeycombstructure can be obtained by extrusion-molding a moldable ceramicmaterial having plasticity by a known die to form a honeycomb green bodyintegrally having an outer peripheral portion and cell walls, cutting,drying and sintering it, removing the outer peripheral portion bymachining, and then coating the exposed outer peripheral surface with acoating material to form an outer peripheral wall.

Particularly for catalyst carriers and filters for diesel engines oflarge vehicles and special vehicles, for example, large ceramichoneycomb structures of 190 mm or more in outer diameter and 200 mm ormore in length, and ceramic honeycomb structures having as thin walls as0.15 mm or less are used. Because such large ceramic honeycombstructures and thin-wall ceramic honeycomb structures are subjected to alarge load by mechanical vibration and impact during use, it is demandedto improve their mechanical strength.

JP 2016-172223 A discloses a honeycomb structure comprising a honeycombstructure body having cell walls partitioning pluralities of cells andintersection portions of the cell walls, each of particular cells amongthe above cells having a polygonal shape with pluralities of convexly orconcavely curved corners in a cross section perpendicular to thelongitudinal direction of the cells, at least one corner being concavelycurved, and the concavely curved corners being 1.5% or more of all cellcorners in the honeycomb structure body. JP 2016-172223 A describes thatsuch structure makes the honeycomb structure resistant to cracking evenin the operation of burning soot, and suppresses the propagation ofcracks, if generated, while suppressing increase in pressure loss.

However, when the structure described in JP 2016-172223 A is used forlarge ceramic honeycomb structures of 190 mm or more in outer diameterand 200 mm or more in length, and ceramic honeycomb structures having asthin walls as 0.15 mm or less, sufficient strength is not obtainedparticularly in their outer peripheral portions, and thus furtherimprovement in strength is desired.

JP 09-299731 A discloses an exhaust gas filter comprising a honeycombcolumn comprising ceramic lattice walls constituting quadrangle cells,the ceramic walls being thicker near ceramic wall intersections than inother portions, and the ceramic walls having substantially circularcross sections at intersections. JP 09-299731 A also discloses that oneof methods for making ceramic walls thicker near intersections than inother portions is to form holes at slit intersections of anextrusion-molding die by drilling. JP 09-299731 A describes that suchstructure can prevent ceramic walls near intersections from beingcracked by thermal stress during the generation of the exhaust gasfilter.

However, when the structure described in JP 09-299731 A is used forlarge ceramic honeycomb structures of 190 mm or more in outer diameterand 200 mm or more in length, and ceramic honeycomb structures having asthin walls as 0.15 mm or less, their strength may not be sufficientparticularly to shock to the outer peripheral walls in handling, etc.,so that further improvement of strength is desired.

JP 2014-46601 A discloses a ceramic honeycomb structure comprisingpluralities of flow paths partitioned by porous ceramic cell walls, eachflow path being provided with corners R having a radius of curvature ina range of 0.01-0.8 mm in a cross section of the ceramic honeycombstructure perpendicular to the flow path direction, and the corners R inthree regions comprising a first region including the center, a secondregion outside the first region, and a third region including an outerperipheral portion outside the second region, having larger averageradii of curvature as getting distant from the center axis toward theouter peripheral portion. JP 2014-46601 A describes that in theproduction of this ceramic honeycomb structure by extrusion moldingusing a die having corners R in accordance with the corners R in thefirst to third regions, the non-uniformity of an extrusion rate issuppressed by adjustment to make extrusion molding resistance smaller asgetting distant from the center portion toward the outer peripheralportion, resulting in less deformation in extrusion molding, andproviding the ceramic honeycomb structure with higher mechanicalstrength because the corners R have larger radii of curvature in theouter peripheral portion.

However, when the die described in JP 2014-46601 A is used toextrusion-mold large ceramic honeycomb structures of 190 mm or more inouter diameter and 200 mm or more in length, and ceramic honeycombstructures having as thin walls as 0.15 mm or less, the resultantceramic honeycomb structures may have insufficient mechanical strength,so that further improvement in strength is desired.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a ceramichoneycomb structure having sufficient mechanical strength, for example,even though it is as large as 190 mm or more in outer diameter and 200mm or more in length, and/or has as thin cell walls as 0.15 mm or less,and a honeycomb-molding die for producing such a ceramic honeycombstructure.

SUMMARY OF THE INVENTION

As a result of intensive research in view of the above object, theinventors have paid attention to the shape of cell wall intersections ina ceramic honeycomb structure having large numbers of flow pathspartitioned by cell walls, finding a structure of providing a ceramichoneycomb structure with improved strength in its outer peripheralportion. The present invention has been completed based on such finding.

Thus, the ceramic honeycomb structure of the present invention has largenumbers of flow paths longitudinally formed by cell walls arranged in alattice pattern in cross section, and an outer peripheral wall formedaround the flow paths;

at least one fan-shaped bulge being projecting in a fan shape toward theflow paths from a cell wall intersection at which the cell walls arecrossing, in a cross section perpendicular to the longitudinaldirection;

circumscribed circles of circular portions of the fan-shaped bulges atall cell wall intersections having a constant radius; and

when the distance between the center point of the circumscribed circleand the center point of the cell wall intersection is defined as acenter point distance S, a center point distance So in an outerperipheral portion of the ceramic honeycomb structure and a center pointdistance Sc in a center portion of the ceramic honeycomb structuremeeting Sc<So.

In the outer peripheral portion of the ceramic honeycomb structure, thecenter point of the circumscribed circle is preferably positioned on theside of the outer peripheral wall of the ceramic honeycomb structurewith respect to the center point of the cell wall intersection.

The center point distance Sc in the center portion of the ceramichoneycomb structure is preferably 20 μm or less, with the differencebetween the center point distance So in the outer peripheral portion andthe center point distance Sc in the center portion being 5-150 μm.

A center point distance Sh of the cell wall intersection in anintermediate portion between the center and outer peripheral portions ofthe ceramic honeycomb structure preferably meets Sc<Sh<So.

The radius of the circumscribed circle is preferably 0.75-1.25 times thethickness of the cell walls of the ceramic honeycomb structure.

The honeycomb-molding die of the present invention comprises supplyholes for supplying a moldable material, and lattice-shaped slits formolding the moldable material to a honeycomb shape, which are formed onthe opposite surface side to a surface side on which the supply holesare formed, with communication with the supply holes;

four corners of a die member at a slit intersection at which thelattice-shaped slits are crossing having circular recesses when viewedin the extrusion direction;

the inscribed circles of the four circular recesses at all slitintersections having a constant radius; and

when the distance between the center point of the inscribed circle andthe center point of the slit intersection is defined as a die centerpoint distance Sd, a die center point distance Sdc in a center portionof the honeycomb-molding die and a die center point distance Sdo in anouter peripheral portion of the honeycomb-molding die meeting Sdc<Sdo.

In the outer peripheral portion of the die, the center point of theinscribed circle is preferably positioned on the outer periphery side ofthe honeycomb-molding die with respect to the center point of the slitintersection.

In the honeycomb-molding die, the die center point distance Sdh at theslit intersection in an intermediate portion between the center andouter peripheral portions of the die preferably meets Sdc<Sdh<Sdo.

Effects of the Invention

The present invention capable of producing ceramic honeycomb structureshaving sufficient mechanical strength is particularly suitable for aslarge ceramic honeycomb structures as 190 mm or more in outer diameterand 200 mm or more in length, and ceramic honeycomb structures having asthin cell walls as 0.15 mm or less. It can also provide ahoneycomb-molding die for producing a ceramic honeycomb structure havingsufficient mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a front view schematically showing an example of the ceramichoneycomb structures of the present invention.

FIG. 1 b is a cross-sectional view schematically showing an example ofthe ceramic honeycomb structures of the present invention in parallelwith its longitudinal direction.

FIG. 2 is a cross-sectional view schematically showing fan-shaped bulgesformed at a cell wall intersection, in a cross section of the ceramichoneycomb structure of the present invention.

FIG. 3 is a schematic view for explaining the positional relationbetween the center point of fan-shaped bulges and the center point of acell wall intersection, in a cross section of the ceramic honeycombstructure of the present invention.

FIG. 4 is a cross section view for explaining an outer peripheralportion of the ceramic honeycomb structure of the present invention.

FIG. 5 is a cross-sectional view showing an example of the ceramichoneycomb structures of the present invention.

FIG. 6 is a schematic view enlargedly showing cell wall intersections ina center portion (A), intermediate portions (G) and (H), and outerperipheral portions (B) to (F) in FIG. 5 .

FIG. 7 is a schematic view showing the positional relations between thecenter points of fan-shaped bulges and the center points of cell wallintersections in the center portion (A) and the outer peripheralportions (B) to (F) in FIG. 5 .

FIG. 8 a is a perspective view showing an example of thehoneycomb-molding dies of the present invention from the slit side.

FIG. 8 b is a perspective view showing an example of thehoneycomb-molding dies of the present invention from the supply holeside.

FIG. 9 is a front view showing an example of the positional relationsbetween slits and supply holes in the honeycomb-molding die of thepresent invention.

FIG. 10 a is a front view showing the honeycomb-molding die of thepresent invention from the slit side.

FIG. 10 b is a cross-sectional view taken along the line P-P in FIG. 10a.

FIG. 11 is a schematic view enlargedly showing slit intersections atpositions A, B, D and E in FIG. 10 a.

FIG. 12 is photomicrographs showing slit intersections at positions A,J, I, D and E in FIG. 10 a.

FIG. 13 a is a combination of a schematic view showing the centerportion (A), and a photomicrograph of cell wall intersections in thatportion, in a cross section of the ceramic honeycomb structure producedin Example 1.

FIG. 13 b is a combination of a schematic view showing outer peripheralportions (E) and (F) (in directions of 45°) and an intermediate portion(I) (in a direction of 45°), and photomicrographs of cell wallintersections in those portions, in a cross section of the ceramichoneycomb structure produced in Example 1.

FIG. 13 c is a combination of a schematic view showing outer peripheralportions (B) and (C) (in a direction of 90°) and an intermediate portion(G) (in a direction of 90°), and photomicrographs of cell wallintersections in those portions, in a cross section of the ceramichoneycomb structure produced in Example 1.

FIG. 14 is a schematic view showing the position and shape of a testpiece cut out for the measurement of B-axis compressive strength.

FIG. 15 is a schematic view showing an example of conventional ceramichoneycomb structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be specifically explainedbelow without intention of restricting the present invention thereto. Itshould be noted that within the scope of the present invention, designmodifications, improvements, etc. may be properly added based on theusual knowledge of those skilled in the art.

[1] Ceramic Honeycomb Structure

As shown in FIGS. 1 a and 1 b , the ceramic honeycomb structure 10 ofthe present invention comprises large numbers of longitudinallyextending flow paths 13 partitioned by cell walls 12 having alattice-shaped cross section, and an outer peripheral wall 11 formedaround the flow paths. In a cross section perpendicular to thelongitudinal direction, as shown in FIG. 2 , a cell wall intersection 14at which the cell walls 12 are crossing has fan-shaped bulges 15 a, 15b, 15 c, 15 d projecting in a fan shape toward the flow paths 13, andthe circumscribed circles 16 of circular portions 16 a, 16 b, 16 c, 16 dof the fan-shaped bulges 15 a, 15 b, 15 c, 15 d at all cell wallintersections 14 have a constant radius. As shown in FIG. 3 , when thedistance between the center point C_(R) of the circumscribed circle 16and the center point C_(W) of the cell wall intersection 14 is definedas a center point distance S, a center point distance So in an outerperipheral portion and a center point distance Sc in a center portion inthe ceramic honeycomb structure 10 meet Sc<So.

In a cross section perpendicular to the longitudinal direction, as shownin FIG. 4 , an x-axis and a y-axis are defined as extending along thecell walls with the axis center O of the honeycomb structure as anorigin, and flow paths 13-2 (four in total) counted second except forthe outermost incomplete flow paths 13-1 from an intersection of eachaxis and the outer peripheral surface toward the origin are selected.Lengths from the origin to four intersections (two on the x-axis, andtwo on the y-axis) of the centerlines of inside (origin-side) cell wallsof the four second flow paths 13-2 and the axes (x-axis and y-axis) areaveraged to obtain an average value r, thereby depicting a circle αhaving a radius r. The outer peripheral portion of the ceramic honeycombstructure is defined as a region Z with the origin as a center outsidethe circle α (on the side of the outer peripheral surface). With thecoordinates of intersections of the cell walls and the axes being a (x₁,0), b (x₂, 0), c (0, y₁), and d (0, y₂), wherein x₁>x₂, and y₁>y₂, theradius r can be calculated by r=(x₁−x₂+y₁−y₂)/4.

The center point distance So in the outer peripheral portion is obtainedby selecting four cell wall intersections in the outer peripheralportion as described below, defining four groups each having three cellwall intersections in total including one selected cell wallintersection and two adjacent cell wall intersections, determining anaverage value Say of their center point distances S in each group, andaveraging them over the above four groups. Four cell wall intersectionsin the outer peripheral portion honeycomb structure are selected at twopositions in a direction along the cell wall (x-axis or y-axis) from theaxis center O (hereinafter referred to as “direction of 90°”), and attwo positions in a direction along the cell wall intersections (diagonallines of flow paths) (hereinafter referred to as “direction of 45°”).Also, two cell wall intersections adjacent to the selected cell wallintersection are defined as those perpendicularly adjacent to an axisalong the cell wall from the center O when they are selected in adirection of the cell wall (direction of 90°), and those on the closerside to the center O along the cell wall intersections when they areselected in a direction of the cell wall intersections (direction of45°).

The center point distance Sc in the center portion is defined as anaverage of center point distances in five cell wall intersections intotal including the closest cell wall intersection to the center of theceramic honeycomb structure, and four cell wall intersections adjacentthereto.

The center point C_(R) of the circumscribed circle 16 is a centerdetermined from circular portions 16 a, 16 b, 16 c, 16 d of fan-shapedbulges 15 a, 15 b, 15 c, 15 d at one cell wall intersection 14, and thecenter point C_(W) of the cell wall intersection 14 is an intersectionof centerlines of two cell walls 12 crossing at one cell wallintersection 14. Specifically, in cell walls 12 a, 12 b, 12 c, 12 dextending in four directions from a cell wall intersection 14 as acenter, as shown in FIG. 3 , middle points M1, M2, M3 and M4 aredetermined in the thickness directions of the cell walls, a lineconnecting the middle points M1 and M3 and a line connecting the middlepoints M2 and M4 are drawn, and their intersection is regarded as thecenter point C_(W) (X_(W), Y_(W)) of the cell wall intersection 14,wherein (X_(W), Y_(W)) are the coordinates of the center point C_(W).Incidentally, each middle point M1, M2, M3, M4 is determined at a middleof the cell wall intersection 14 and its adjacent cell wallintersection. In one cell wall intersection 14, a center of acircumscribed circle 16 in contact with the contours of circularportions 16 a, 16 b, 16 c, 16 d of four fan-shaped bulges 15 a, 15 b, 15c, 15 d is regarded as the center point C_(R) (X_(R), Y_(R)) of thecircumscribed circle 16, wherein (X_(R), Y_(R)) represents thecoordinates of the center point C_(R). In some cell wall intersections14, only three or two fan-shaped bulges may be clearly projecting towardthe flow paths 13. In that case, a circumscribed circle 16 is drawn fromthe circular portions of three or two fan-shaped bulges, and its centeris determined as the center point C_(R) (X_(R), Y_(R)) of thecircumscribed circle 16.

The words that the circumscribed circles 16 at all cell wallintersections 14 have a constant radius mean that their radii are equalwithin a range of variation which may occur in the production. Thoughceramic honeycomb structures are designed such that the circumscribedcircles of circular portions of fan-shaped bulges have a constantradius, slight deformation may occur in cell walls and fan-shaped bulgesproduced, resulting in varied radii of the circumscribed circles fromposition to position. Accordingly, in the present invention, the radiiof the circumscribed circles are regarded as equal as long as they arewithin this range of variation. The variation range of the radii of thecircumscribed circles is within ±5%.

With circumscribed circles 16 of the circular portions 16 a, 16 b, 16 c,16 d of the fan-shaped bulges 15 a, 15 b, 15 c, 15 d at all cell wallintersections 14 having a constant radius, the total area of fan-shapedbulges 15 a, 15 b, 15 c, 15 d in the center portion is substantially thesame as the total area of fan-shaped bulges 15 a, 15 b, 15 c, 15 d inthe outer peripheral portion in a radial cross section, the amount of amoldable material supplied to an extrusion-molding die comprising supplyholes for supplying a moldable material and lattice-shaped slitscommunicating with the supply holes does not substantially differbetween the center portion and the outer peripheral portion duringextrusion molding, avoiding the generation of molding strain between thecenter portion and the outer peripheral portion, and thus suppressingcracking in sintering.

The center point distance S is a distance between the center point C_(W)and the center point C_(R), which is determined byS=[(X_(W)−X_(R))²⁺(Y_(W)−Y_(R))²]^(1/2). The coordinates (X_(W), Y_(W))of the center point C_(W) and the coordinates (X_(R), Y_(R)) of thecenter point C_(R) may be determined with an arbitrary point, forexample, the center point of the ceramic honeycomb structure 10, as anorigin. These center points C_(W) and C_(R) can be determined bymeasuring a cross section of the ceramic honeycomb structure 10, forexample, by an image analyzer (Quick Vision available from MitutoyoCorporation).

A cross section of the ceramic honeycomb structure 10 of the presentinvention is shown in of FIG. 5 , and cell wall intersections 14 in acenter portion (A) and outer peripheral portions (B) to (F) in FIG. 5are enlargedly shown in FIGS. 6 (A) to 6 (F), respectively. At each cellwall intersection 14 in the center portion (A) and the outer peripheralportions (B) to (F) shown in FIG. 5 , the center point distance Sbetween the center point C_(R) of the circumscribed circles 16 of thecircular portions 16 a, 16 b, 16 c, 16 d of the fan-shaped bulges 15 a,15 b, 15 c, 15 d and the center point C_(W) of the cell wallintersection 14, and its direction are shown in FIGS. 7 (A) to 7 (F). Asshown in FIG. 7 (A), in the center portion (A) of the ceramic honeycombstructure 10, the center point C_(R) of the circumscribed circle 16 ispreferably positioned at the center point C_(W) of the cell wallintersection 14.

In the center portion (A) of the ceramic honeycomb structure 10,however, the center point C_(R) of the circumscribed circle 16 isactually not in complete accordance with the center point C_(W) of thecell wall intersection 14, because their positions are variabledepending on the production conditions, etc. of the ceramic honeycombstructure 10. Accordingly, the center point distance S_(C) between thecenter point C_(R) of the circumscribed circle 16 and the center pointC_(W) of the cell wall intersection 14 in the center portion (A) isexpressed by an average of values measured at five cell wallintersections in total including a cell wall intersection closest to thecenter of the ceramic honeycomb structure 10, and four cell wallintersections adjacent to the closest cell wall intersection.

In the present invention, the center point distance So in the outerperipheral portion [for example, average in four outer peripheralportions (C), (D), (E) and (F)] is larger than the center point distanceSc in the center portion (A), namely Sc<So.

In the honeycomb structure shown in FIG. 5 , for example, the outerperipheral portions (C) and (D) in the direction of 90° and the outerperipheral portions (E) and (F) in the direction of 45° are selected toexplain a method for determining a center point distance So. First,center point distances S at three cell wall intersections in total,including a cell wall intersection in the outer peripheral portion (C)and two cell wall intersections (C2) and (C3) adjacent thereto, aremeasured and averaged. At cell wall intersections in the outerperipheral portions (D), (E) and (F), too, center point distances atthree cell wall intersections in total, including one selected cell wallintersection and two cell wall intersections [(D2) and (D3), (E2) and(E3), and (F2) and (F3), respectively] adjacent thereto in each outerperipheral portion, are similarly measured and averaged. Average centerpoint distances in these four outer peripheral portions (C), (D), (E)and (F) are further averaged to obtain the center point distance So.

In the outer peripheral portion, the center point C_(R) of thecircumscribed circle 16 is preferably separate from the center pointC_(W) of the cell wall intersection 14 toward the outer peripheral wall11 of the ceramic honeycomb structure 10 by the center point distanceSo. The term “toward the outer peripheral wall 11” means toward an outerperipheral wall 11 closest to the cell wall intersection 14, namely, onthe opposite side to the center axis of the ceramic honeycomb structure.

It is preferable that the fan-shaped bulges 15 a, 15 b, 15 c, 15 d areformed substantially uniformly in four flow paths in the center portion(A) as shown in FIG. 7 (A), while in the outer peripheral portion (C) inthe direction of 90°, the fan-shaped bulges 15 a, 15 d formed on theside of the outer peripheral wall 11 (left side in the figure) havelarger areas than those of the fan-shaped bulges 15 b, 15 c formed onthe side of the center axis of the ceramic honeycomb structure 10 (rightside in the figure) as shown in FIG. 7 (C).

Likewise, in the outer peripheral portion (B) in the direction of 90°,as shown in FIG. 7 (B), the fan-shaped bulges 15 b, 15 c formed on theside of the outer peripheral wall 11 (right side in the figure)preferably have larger areas than those of the fan-shaped bulges 15 a,15 d formed on the center side (left side in the figure).

Further, in the outer peripheral portion (D) in the direction of 90°, asshown in FIG. 7 (D), the fan-shaped bulges 15 c, 15 d formed on the sideof the outer peripheral wall 11 (lower side in the figure) preferablyhave larger areas than those of the fan-shaped bulges 15 a, 15 b formedon the center side (upper side in the figure).

Further, in the outer peripheral portion (E) in the direction of 45°from the center axis of the ceramic honeycomb structure, as shown inFIG. 7 (E), the fan-shaped bulge 15 c formed on the side of the outerperipheral wall 11 (lower right side in the figure) preferably has alarger area than that of the fan-shaped bulge 15 a formed on the centerside (upper left side in the figure), with the fan-shaped bulges 15 b,15 d formed on the upper right and lower left sides of the cell wallintersection 14 having intermediate areas of them.

Likewise, in the outer peripheral portion (F) in the direction of 45°from the center axis of the ceramic honeycomb structure, as shown inFIG. 7 (F), the fan-shaped bulge 15 d formed on the side of the outerperipheral wall 11 (lower left side in the figure) has a larger areathan that of the fan-shaped bulge 15 b formed on the center side (upperright side in the figure), with the fan-shaped bulges 15 a, 15 c formedon the upper left and lower right sides of the cell wall intersection 14having intermediate areas of them.

Thus, with the center point distance So in the outer peripheral portionof the ceramic honeycomb structure and the center point distance Sc inthe center portion meeting Sc<So, and the center point C_(R) of thecircumscribed circle 16 being separate from the center point C_(W) ofthe cell wall intersection 14, toward the outer peripheral wall 11 fromthe center axis of the ceramic honeycomb structure 10 by the centerpoint distance So in the outer peripheral portion, the extrusion-moldedceramic honeycomb green body and the ceramic honeycomb sintered bodyobtained by drying and sintering the ceramic honeycomb green body arereinforced by the fan-shaped bulges 15 a, 15 b, 15 c, 15 d formed withdifferent areas at the cell wall intersections 14 in the outerperipheral portion, having higher resistance to cracking. Also, even ifcracks were generated, they would not propagate into the ceramichoneycomb structure 10. Among the fan-shaped bulges 15 a, 15 b, 15 c, 15d formed with different areas at cell wall intersections 14 in the outerperipheral portion, those having large areas presumably contribute toreinforcement. Particularly when receiving shock from a surroundingsurface, the fan-shaped bulges formed with larger areas on the side ofthe outer peripheral wall 11 can well reinforce the cell wallintersections 14. Thus, the outer peripheral portion preferably hashigher mechanical strength than in a ceramic honeycomb structure inwhich the center point distance So in an outer peripheral portion is thesame as the center point distance Sc in a center portion. Further,because the total area of fan-shaped bulges 15 a, 15 b, 15 c, 15 d doesnot substantially differ between the center and outer peripheralportions of the ceramic honeycomb structure, the center and outerperipheral portions have substantially the same opening ratio, desirablyavoiding pressure loss increase as compared with a ceramic honeycombstructure in which the center point distance So in an outer peripheralportion is the same as the center point distance Sc in a center portion.

In the present invention, the center point distance Sc in the centerportion (A) is preferably 20 μm or less. Further, the center pointdistance S between the center point C_(R) of the circumscribed circle 16and the center point C_(W) of the cell wall intersection 14 preferablyincreases as the cell wall intersection 14 becomes distant from thecenter of the ceramic honeycomb structure 10. In this case, thedifference between the center point distance Sc in the center portion ofthe ceramic honeycomb structure 10 and the center point distance So inthe outer peripheral portion is preferably 5-150 μm. The differencebetween Sc and So is preferably 10-120 μm, and further preferably 10-100μm. With such center point distance Sc and such difference between Scand So, the cell wall intersections are well reinforced, so that cracksreceived from the side of the outer peripheral wall 11, if any, wouldnot easily propagate inside.

At a position of X (mm) from the axis center of the ceramic honeycombstructure 10, the center point distance S (μm) is further preferablyexpressed by S=F₁×X+M₁, wherein F₁ is a constant of 0.05-0.4, and M₁ isa constant of 0-20. Because M₁ represents the center point distance Scin the center portion, the difference between the center point distanceSc in the center portion and the center point distance So in the outerperipheral portion is expressed by the formula of So−Sc=F₁×ΔX, whereinΔX is the distance (mm) from the axis center to the outer peripheralportion.

In an intermediate portion between the center and outer peripheralportions of the ceramic honeycomb structure, a center point distance Shat a cell wall intersection preferably meets Sc<Sh<So. The intermediateportion is as distant from the center portion of the ceramic honeycombstructure as about half of the distance between the center portion andthe outer peripheral portion. The center point distance Sh is a valueobtained by selecting four cell wall intersections (two in thedirections of 90° and two in the directions of 45°) in the intermediateportion, defining four groups each having three cell wall intersectionsin total including one selected cell wall intersection [for example, (G)or (H) in FIG. 5 ] and two adjacent cell wall intersections [(G2) and(G3), or (H2) and (H3)], obtaining an average of center point distancesS in each group, and averaging them over the four groups. The shapes offan-shaped bulges 15 a, 15 b, 15 c, 15 d at cell wall intersections 14in the center portion (A), intermediate portions (G) and (H) (middleportions between the center portion and the outer peripheral portion),and outer peripheral portions (B) and (C) of the ceramic honeycombstructure in FIG. 5 are shown in FIGS. 6 (A), 6 (G), 6 (H), 6 (B) and 6(C), respectively.

For example, among fan-shaped bulges 15 a, 15 b, 15 c, 15 d formed atcell wall intersections 14 in the center portion (A), intermediateportion (H) and outer peripheral portion (C) of the ceramic honeycombstructure, the areas of the fan-shaped bulges 15 a, 15 d formed on theside of the outer peripheral wall 11 close to the outer peripheralportion (C) (left side in the figure) get larger as their positionsshift from the center portion (A) to the intermediate portion (H) andthe outer peripheral portion (C) in this order, while the areas of thefan-shaped bulges 15 b, 15 c formed on the opposite side (right side inthe figure) get smaller in this order.

Likewise, in the center portion (A), intermediate portion (G) and outerperipheral portion (B) of the ceramic honeycomb structure, too, theareas of fan-shaped bulges 15 b, 15 c formed on the side of the outerperipheral wall 11 close to the outer peripheral portion (B) (right sidein the figure) get larger as their positions shift from the centerportion (A) to the intermediate portion (G) and the outer peripheralportion (B) in this order, while the areas of fan-shaped bulges 15 a, 15d formed on the opposite side (left side in the figure) get smaller inthis order.

Such structure reinforces fan-shaped bulges closer to the outerperipheral wall 11 in the outer peripheral and intermediate portionsthan a ceramic honeycomb structure in which the center point distance Soin an outer peripheral portion is the same as the center point distanceSc in a center portion, resulting in less cracking and higher mechanicalstrength.

The radius of the circumscribed circle 16 is preferably 0.75-1.25 times,more preferably 0.8-1.1 times, the thickness of the cell walls of theceramic honeycomb structure 10. The radius of the circumscribed circle16 is preferably 230-380 μm, and more preferably 240-330 μm, thoughvariable depending on the thickness of the cell walls.

The ceramic honeycomb structure of the present invention preferably hasan average cell wall thickness of 4-15 mil (0.102-0.381 mm), and anaverage cell density of 150-300 cpsi (23.3-46.5 cells/cm²). With suchcell wall structure, even large and/or thin-wall ceramic honeycombstructures can have sufficient mechanical strength. The average cellwall thickness of less than 4 mil provides the cell walls with lowstrength, while the average cell wall thickness of more than 15 milmakes it difficult to keep low pressure loss in an early period of use.The average cell density of less than 150 cpsi provides the cell wallswith low strength, while the average cell density of more than 300 cpsimakes it difficult to keep low pressure loss.

Because ceramic honeycomb structures are used for cleaning exhaust gasesdischarged from diesel engines, materials for cell walls of the ceramichoneycomb structure of the present invention are preferablyheat-resistant ceramics, namely, ceramics comprising alumina, mullite,cordierite, silicon carbide, silicon nitride, zirconia, aluminumtitanate, lithium aluminum silicate, etc. as main crystals. Preferableamong them are ceramics comprising low-thermal-expansion cordieritehaving excellent heat shock resistance as a main crystal.

[2] Production Method of Ceramic Honeycomb Structure

A method for producing the ceramic honeycomb structure of the presentinvention comprises the steps of (a) extrusion-molding a moldablematerial containing ceramic materials to a predetermined green bodycomprising an outer peripheral portion integral with cell walls, and (b)drying and sintering the green body to obtain a sintered body;

the above extrusion molding step using a honeycomb-molding diecomprising supply holes for supplying the moldable material, andlattice-shaped slits for molding the moldable material to a honeycombshape, which are formed on the opposite surface side to a surface sidehaving the supply holes, with communication with the supply holes;

each of four corners of a die member having a circular recess at eachslit intersection at which the lattice-shaped slits are crossing, whenviewed in the extrusion direction;

the inscribed circles of the four circular recesses at all slitintersections having a constant radius;

when the distance between the center point of the inscribed circle andthe center point of the slit intersection is defined as a die centerpoint distance Sd, a die center point distance Sdc in the center portionof the honeycomb-molding die and a die center point distance Sdo in theouter peripheral portion of the honeycomb-molding die meeting Sdc<Sdo.

After the step (b) of drying and sintering the green body to obtain thesintered body, if necessary, a step (c) of coating an outer peripheralsurface of the sintered body with or without removal of its outerperipheral portion by machining with a coating material andheat-treating the coating material to form an outer peripheral wall maybe conducted.

A ceramic honeycomb green body obtained by extrusion-molding themoldable ceramic material by this die comprises large numbers oflongitudinal flow paths formed by cell walls arranged in a latticepattern, in a cross section perpendicular to the longitudinal direction;at least one fan-shaped bulge being projecting in a fan shape toward theflow paths from a cell wall intersection at which the cell walls arecrossing; the circumscribed circles of circular portions of thefan-shaped bulges at all cell wall intersections having a constantradius; and when the distance between the center point of thecircumscribed circle and the center point of the cell wall intersectionis defined as a center point distance S, a center point distance So inthe outer peripheral portion of the ceramic honeycomb structure and acenter point distance Sc in the center portion meeting Sc<So. By dryingand sintering this ceramic honeycomb green body, and forming an outerperipheral wall, the ceramic honeycomb structure of the presentinvention can be obtained.

(1) Ceramic Materials

Ceramic materials are preferably prepared to a cordierite-formingmaterial. The cordierite-forming material is obtained by mixing powderymaterials comprising silica sources, alumina sources and magnesiasources, such that its main crystal is cordierite having a chemicalcomposition comprising 42-56% by mass of SiO₂, 30-45% by mass of Al₂O₃,and 12-16% by mass of MgO. Because pores formed in ceramics comprisingcordierite as a main crystal are formed by the melting of the ceramicmaterials in the sintering process, the structure of pores formed bysintering cordierite-based ceramics can be controlled by adjusting theparticle sizes and particle size distributions of ceramic materials suchas kaolin, silica, talc, alumina, etc.

(2) Honeycomb-Molding Die

As shown in FIGS. 8 a and 8 b , the molding die 20 comprises supplyholes 22 for supplying a moldable material, and lattice-shaped slits 21for molding the moldable material to a honeycomb shape, which are formedon the opposite surface side to a surface side on which the supply holes22 are formed with communication with the supply holes 22. The moldableceramic material introduced into the molding die 20 through the supplyholes 22 is molded by the slits 21 to a honeycomb shape. Eachquadrilateral cross section of the die member 26 encircled by the slits21 corresponds to each cross section of the flow paths of the honeycombstructure. All supply holes 22 are communicating with all intersectionsof the lattice-arranged slits 21, or the supply holes 22 arecommunicating with the intersections of the slits 21 in a checkerboardpattern (shown in FIG. 9 ).

The molding die 20 of the present invention is shown in FIGS. 10 a and10 b , and slit intersections at positions A, B, D and E in the moldingdie 20 shown in FIG. 10 a are enlargedly shown in FIGS. 11 (A), (B), (D)and (E). The molding die 20 of the present invention comprises circularrecesses 24 a, 24 b, 24 c, 24 d at four corners 26 a, 26 b, 26 c, 26 dof the die member positioned at a slit intersection 23 at which thelattice-shaped slits 21 are crossing, when viewed in the extrusiondirection, and the inscribed circles 24 of the four circular recesses 24a, 24 b, 24 c, 24 d at all slit intersections have a constant radius. Aguide ring 29 (having a circular inner contour for a circular honeycombstructure as shown in FIG. 10 a ) for determining the outer diameter ofthe honeycomb structure and forming an outer peripheral wall is arrangedon a surface having slits 21 in an outer peripheral portion of the diemember 26. The inner contour of the guide ring 29 determines the outerperiphery 25 of the die member 26.

When the distance between the center point Ca of the inscribed circle 24and the center point Cs of the slit intersection 23 is defined as a diecenter point distance Sd, a die center point distance Sdc in the centerportion of the honeycomb-molding die 20 and a die center point distanceSdo in the outer peripheral portion of the honeycomb-molding die meetSdc<Sdo.

The die center point distance Sdc in the center portion when viewed inthe extrusion direction is an average of die center point distances atfive slit intersections in total, including a slit intersection closestto the center of the honeycomb-molding die, and four adjacent slitintersections. As shown in FIG. 10 a , with an x-axis and an y-axisextending along the slits 21 from the center Od of the die as an originwhen viewed in the extrusion direction, the outer peripheral portion isdefined as a region Zd outside a circle β having a radius rd with theorigin as a center, wherein rd is determined by counting third andfourth slit intersections 21-3, 21-4 on or closest to each axis from anintersection of each axis and the outer periphery 25, determining middlepoints (two on the x-axis, and two on the y-axis) of the third andfourth slit intersections 21-3, 21-4 on the axes to know lengths fromthe origin to the middle points, and averaging the lengths. The diecenter point distance Sdo in the outer peripheral portion is defined asa value determined by selecting four slit intersections in the outerperipheral portion, defining four groups each having three slitintersections in total including one selected slit intersection and twoadjacent slit intersections, obtaining an average of die center pointdistances in each group, and averaging them over the four groups.

In the center portion of the honeycomb-molding die 20 (at A in FIG. 10 a), as shown in FIG. 11 (A), it is preferable to design that the centerpoint Ca of the inscribed circle 24 is positioned at the center point Csof the slit intersection 23. However, even in such design of thehoneycomb-molding die 20, the center point Ca is actually notnecessarily in complete accordance with the center point Cs.Accordingly, the die center point distance Sdc in the center portion ofthe die (at A in FIG. 10 a ) is expressed by an average of valuesmeasured at five slit intersections including a slit intersectionclosest to the die center and four adjacent slit intersections.

At positions (B, D and E in FIG. 10 a ) in the outer peripheral portionof the honeycomb-molding die 20, as shown in FIGS. 11 (B), 11 (D) and 11(E), the center point Ca of the inscribed circle 24 is separate from thecenter point Cs of the slit intersection 23 toward the outer periphery25 (in radial directions from the center of the honeycomb-molding die20). For example, at the position E, the center point Ca of theinscribed circle 24 is separate from the center point Cs of theintersection 23 toward the outer periphery 25, along a line connectingthe center of the honeycomb-molding die 20 (the center point Cs of theintersection 23 at the position A) and the center point Cs of theintersection 23 at the position E.

In the outer peripheral portions (B, D, E and F in FIG. 10 a ) of thedie, the die center point distance Sdo between the center point Ca andthe center point Cs is a value obtained by selecting four slitintersections in the outer peripheral portion, defining four groups eachhaving three slit intersections in total including one selected slitintersection and two adjacent slit intersections, determining an averageof die center point distances in each group, and averaging them over thefour groups. Thus, with four slit intersections selected in the outerperipheral portion, and each having three slit intersections in totalincluding one selected slit intersection and two adjacent slitintersections, an average of die center point distances is obtained ateach selected slit intersection, and the die center point distance Sdois determined by averaging the resultant average values. The four slitintersections selected in the outer peripheral portion are two inparallel with the slits (hereinafter called “direction of 90°”) and twoin diagonal directions of the slit (hereinafter called “direction of45°”), from the center of the honeycomb-molding die 20.

In the die shown in FIG. 10 a , for example, outer peripheral portions Band D in the directions of 90° and outer peripheral portions E and F inthe directions of 45° are selected to explain a method for determiningthe die center point distance Sdo. First, die center point distances aremeasured at three slit intersections in total including one slitintersection (B) and two adjacent slit intersections (B2 and B3) in theouter peripheral portion to determine their average value. At each ofslit intersections in the outer peripheral portions D, E and F, too, diecenter point distances are similarly measured at three slitintersections in total including each slit intersection (D, E, F) andtwo adjacent slit intersections (D2 and D3, E2 and E3, and F2 and F3,respectively), and averaged. These four average values of die centerpoint distances in the outer peripheral portions B, D, E and F arefurther averaged to determine the die center point distance Sdo.

In intermediate portions of the die (at I and J in FIG. 10 a ) betweenthe center and outer peripheral portions of the die, the die centerpoint distances Sdh of slit intersections preferably meet Sdc<Sdh<Sdo.In this case, the die center point distance Sdo in the outer peripheralportion of the die is preferably 1.5-3 times, more preferably 1.7-2.5times, the die center point distance Sdc in the center portion of thedie. The die center point distance Sdh in the intermediate portion ofthe die is preferably 1.1-2.5 times, more preferably 1.3-2.0 times, thedie center point distance Sdc in the center portion of the die.

The intermediate portion of the die is separate from the center portionof the die by about half of the distance between the center and outerperipheral portions of the die. Also, the die center point distance Sdhat a slit intersection in the intermediate portion of the die isdetermined by selecting four slit intersections in the intermediateportion of the die, defining four groups each having three slitintersections in total including one selected slit intersection and twoadjacent slit intersections, determining an average of die center pointdistances in each group, and averaging them over the four groups, as inthe outer peripheral portion.

The center point distance Sd (μm) at a position separate from the centerof the honeycomb-molding die 20 by X (mm) is further preferablyexpressed by Sd=F₂×X+M₂, wherein F₂ is a constant of 0.05-0.5, and M₂ isa constant of 0-30. Namely, the center point distance further preferablyincreases toward the outer periphery 25 in proportion to the distancefrom the center of the honeycomb-molding die 20.

Though the diameter of the inscribed circle 24 need only be larger thanthe width of the slit 21, it is preferably 1.5-2.5 times, morepreferably 1.6-2.2 times, the width t of the slit 21.

Such honeycomb-molding die 20 can be produced by forming the supplyholes 22, and then forming holes 240 and the slits 21 on the oppositesurface side to a surface side on which the supply holes 22 are formed.In this case, the center of each hole 240 corresponds to the centerpoint Ca of each inscribed circle 24, so that four corners of the diemember at each slit intersection 23 of the honeycomb-molding die 20 areprovided with circular recesses when viewed in the extrusion direction.The holes and the slits 21 are formed, such that the center of each hole240 (corresponding to the center point Ca of each inscribed circle 24)is separate from the center point Cs of each intersection 23 by thecenter point distance Sd toward the outer periphery 25 from the centerof the honeycomb-molding die 20. The holes 240 can be formed by drillingwith position control, for example, by a precise XY-stage drillingmachine.

(3) Coating Material

A coating material, which is applied if necessary to an outer peripheralsurface of the sintered body with or without the removal of its outerperipheral portion by machining, preferably comprises ceramic aggregateparticles and an inorganic binder as main components. In the coatingmaterial, the ceramic aggregate particles are used as aggregates, andthe inorganic binder is used to bind the aggregates. By applying thecoating material comprising the ceramic aggregate particles and theinorganic binder as main components to the outer peripheral surface ofthe ceramic honeycomb body whose outer peripheral portion is removed bymachining, and heat-treating it, the resultant outer peripheral wall iswell bonded to the outer peripheral surface, resulting in a ceramichoneycomb structure having good heat shock resistance with lesscracking.

Because the ceramic honeycomb structure of the present invention is usedfor exhaust-gas-cleaning apparatuses, the ceramic aggregate particlesused in the coating material are preferably made of at least oneselected from the group consisting of cordierite, silica, alumina,mullite, silicon carbide, silicon nitride, and aluminum titanate, whichhave good heat resistance. Among them, cordierite and/or silica reducethe thermal expansion coefficient of the outer peripheral wall,providing higher heat shock resistance.

EXAMPLES

The present invention will be explained in further detail by Examples,without intention of restricting the scope of the present inventionthereto.

Examples 1-3

A Die 20 shown in FIGS. 8 a, 8 b , 9, 10 a, 10 b and 11 was produced asan extrusion-molding die. The die 20 comprised supply holes 22 forsupplying a moldable material, and lattice-shaped slits 21 for moldingthe moldable material to a honeycomb shape, which are formed on theopposite surface side to a surface side, on which the supply holes 22were formed, with communication with the supply holes 22. Each slitintersection 23 of the slits 21 (300 μm in width and 1.58 mm in pitch)was provided with circular recesses 24 a, 24 b, 24 c, 24 d of 540 μm indiameter. In this die 20, the center point Ca of an inscribed circle 24of the circular recesses 24 a, 24 b, 24 c, 24 d was separate from thecenter point Cs of the slit intersection 23 toward the outer periphery25 from the center of the honeycomb-molding die 20 (in a radialdirection from the center of the honeycomb-molding die 20), by thecenter point distance Sd (μm) expressed by the formula of Sd(μm)=0.14×X+17, wherein X is a radial distance (mm) from the center ofthe die. The inner diameter of the outer periphery 25 was 277 mm inExamples 1 and 2, and in 239 mm Example 3.

FIG. 12 is a set of photomicrographs showing slit intersections 23 inthe die of Example 1. FIG. 12 (A) shows a slit intersection positionedin the center portion (at A in FIG. 10 a ) of the die, FIG. 12 (E) showsa slit intersection positioned in the outer peripheral portion (at E inFIG. 10 a ) of the die, FIG. 12 (I) shows a slit intersection positionedin the intermediate portion (at I in FIG. 10 a ) between the slitintersection shown in FIG. 12 (A) and the slit intersection shown inFIG. 12 (E), FIG. 12 (D) shows a slit intersection positioned in theouter peripheral portion (at D in FIG. 10 a ), and FIG. 12 (J) shows aslit intersection positioned in the intermediate portion (at J in FIG.10 a ) between the slit intersection shown in FIG. 12 (A) and the slitintersection shown in FIG. 12 (D).

The center point distances Sd of slit intersections (between the centerpoint Ca of the inscribed circle 24 and the center point Cs of the slitintersection 23) in the center, outer peripheral and intermediateportions of the die of Example 1 were determined by the method describedbelow. In the center portion (A in FIG. 10 a ), the center pointdistances Sd was determined by averaging values measured at one slitintersection closest to the axis center and four adjacent slitintersections (five in total). In the outer peripheral portion, thecenter point distances Sd was determined by selecting four slitintersections B, D, E and F in FIG. 10 a , defining four groups eachhaving three slit intersections in total including one selected slitintersection B, D, E, F and two adjacent slit intersections (B2 and B3,D2 and D3, E2 and E3, and F2 and F3, respectively), obtaining an averageof center point distances in each group, and averaging them over thefour groups. In the intermediate portion, the center point distances Sdwas determined by selecting four slit intersections in total including Iand J in FIG. 10 a and other two (not shown), defining four groups eachhaving three slit intersections in total including one selected slitintersection and two adjacent slit intersections (I2 and I3, and J2 andJ3, respectively), obtaining an average of center point distances ineach group, and averaging them over the four groups. The results areshown in Table 1.

The center point distances Sd of slit intersections (between the centerpoint Ca of the inscribed circle 24 and the center point Cs of the slitintersection 23) in the center, outer peripheral and intermediateportions of the dies of Examples 2 and 3 were determined in the samemanner as in Example 1. The results are shown in Table 1.

TABLE 1 Distance Center Point Die Position of Slit from Center DistanceSd D⁽²⁾ Positional Relation Structure Intersections⁽¹⁾ (mm) (μm) (μm) ofCa to Cs Example Center Portion (A) of 0.9 18.0  0.0 — 1 DieIntermediate Portion (I) 67 27.0 — Outer Periphery in Direction of 45°Side⁽³⁾ Intermediate Portion in 67 27.2 — Outer Periphery Direction of45° Side⁽³⁾ Intermediate Portion (J) 67 25.0 — Outer Periphery inDirection of 90° Side⁽³⁾ Intermediate Portion in 67 24.9 — OuterPeriphery Direction of 90° Side⁽³⁾ Average in Intermediate 67 26.0  8.0— Portion Outer Peripheral Portion 133 36.0 — Outer Periphery (E) inDirection of 45° Side⁽³⁾ Outer Peripheral Portion 133 36.9 — OuterPeriphery in Direction of 45° Side⁽³⁾ Outer Peripheral Portion 133 35.0— Outer Periphery (D) in Direction of 90° Side⁽³⁾ Outer PeripheralPortion 133 34.2 — Outer Periphery in Direction of 90° Side⁽³⁾ Averagein Outer 133 35.5 17.5 — Peripheral Portion Example Center Portion (A)of 0.9 6.2  0.0 — 2 die Intermediate Portion (I) 67 25.3 — OuterPeriphery in Direction of 45° Side⁽³⁾ Intermediate Portion in 67 23.9 —Outer Periphery Direction of 45° Side⁽³⁾ Intermediate Portion (J) 6721.8 — Outer Periphery in Direction of 90° Side⁽³⁾ Intermediate Portionin 67 25.0 — Outer Periphery Direction of 90° Side⁽³⁾ Average inIntermediate 67 24.0 17.8 — Portion Outer Peripheral Portion 133 38.8 —Outer Periphery (E) in Direction of 45° Side⁽³⁾ Outer Peripheral Portion133 32.7 — Outer Periphery in Direction of 45° Side⁽³⁾ Outer PeripheralPortion 133 34.1 — Outer Periphery (D) in Direction of 90° Side⁽³⁾ OuterPeripheral Portion 133 32.1 — Outer Periphery in Direction of 90°Side⁽³⁾ Average in Outer 133 34.4 28.2 — Peripheral Portion ExampleCenter Portion (A) of 0.9 5.4  0.0 — 3 Die Intermediate Portion (I) 5621.2 — Outer Periphery in Direction of 45° Side⁽³⁾ Intermediate Portionin 56 19.6 — Outer Periphery Direction of 45° Side⁽³⁾ IntermediatePortion (J) 56 17.7 — Outer Periphery in Direction of 90° Side⁽³⁾Intermediate Portion in 56 17.0 — Outer Periphery Direction of 90°Side⁽³⁾ Average in Intermediate 56 18.9 13.5 — Portion Outer PeripheralPortion 110 26.6 — Outer Periphery (E) in Direction of 45° Side⁽³⁾ OuterPeripheral Portion 110 27.8 — Outer Periphery in Direction of 45°Side⁽³⁾ Outer Peripheral Portion 110 22.9 — Outer Periphery (D) inDirection of 90° Side⁽³⁾ Outer Peripheral Portion 110 27.2 — OuterPeriphery in Direction of 90° Side⁽³⁾ Average in Outer 110 26.1 20.7 —Peripheral Portion Note: ⁽¹⁾The Symbols (A), (D), (E), (I) and (J)express the positions of slit intersections in FIG. 10a, and thosewithout symbols are not shown in FIG. 10a. Note: ⁽²⁾D represents thedifference determined by subtracting the center point distance Sd in thecenter portion of the die from an average of center point distances Sdin various portions. Note: ⁽³⁾The center point Ca of the inscribedcircle was deviated from the center point Cs of the slit intersectiontoward the outer periphery.

Kaolin powder, talc powder, silica powder and alumina powder were mixedto provide a cordierite-forming material powder comprising 50% by massof SiO₂, 36% by mass of Al₂O₃ and 14% by mass of MgO, which wassufficiently dry-mixed with methylcellulose and hydroxypropylmethylcellulose as a binder, a lubricant, and foamed resin balloons as apore-forming material, and then sufficiently blended with apredetermined amount of water to prepare a plasticized moldable ceramicmaterial.

The moldable ceramic material was extrusion-molded by the above die, andcut to a predetermined length to obtain a ceramic honeycomb green body.This green body was dried, and then sintered at 1410° C. to obtain acordierite-based ceramic honeycomb sintered body. After removing anouter peripheral portion from this ceramic honeycomb sintered body bymachining, an exposed outer peripheral surface of the ceramic honeycombbody was coated with an outer peripheral wall material comprisingamorphous silica, colloidal silica and water, and then dried to producea cordierite-based ceramic honeycomb structure (outer diameter: 266.7mm, length: 304.8 mm, cell wall thickness: 0.3 mm, cell pitch: 1.58 mm,and cell wall porosity: 61% in Examples 1 and 2, and outer diameter:228.6 mm, length: 254 mm, cell wall thickness: 0.3 mm, cell pitch: 1.58mm, and cell wall porosity: 61% in Example 3).

FIGS. 13 a, 13 b and 13 c show the optical microscopic observationresults of a cross section of the ceramic honeycomb structure of Example1 perpendicular to the longitudinal direction. FIG. 13 a shows cell wallintersections at the position (A) in the center portion. FIG. 13 b showscell wall intersections at the position (I) 67 mm distant from thecenter axis in the intermediate portion, and at the positions (E) and(F) 133 mm distant from the center axis in the outer peripheral portion,in directions from the center axis to cell wall intersections(directions of 45°). FIG. 13 c shows cell wall intersections at theposition (G) 67 mm distant from the center axis in the intermediateportion, and at the positions (B) and (C) 133 mm distant from the centeraxis in the outer peripheral portion, in directions along the cell wallfrom the center axis (directions of 90°). These results indicate thatthe cell wall intersections of the ceramic honeycomb structure ofExample 1 have fan-shaped bulges projecting in a fan shape toward theflow paths.

The circumscribed circles of circular portions of fan-shaped bulges weremeasured on the optical photomicrographs of these cell wallintersections by an image analyzer (Quick Vision available from MitutoyoCorporation), to determine their radii, the center point distance Sbetween the center point C_(R) of each circumscribed circle and thecenter point C_(W) of each cell wall intersection, their positionalrelations, and ratios of the radius of the circumscribed circle to thecell wall thickness. The center point distance in the center portion wasexpressed by an average value of center point distances in five cellwall intersections in total including one cell wall intersection nearthe center and four adjacent cell wall intersections, and each of thecenter point distances in the outer peripheral and intermediate portionswas expressed by a value determined by selecting four cell wallintersections in total at two positions in the directions of 90° and twopositions in the directions of 45°, defining four groups each havingthree cell wall intersections in total including one selected cell wallintersection and two adjacent cell wall intersections (B2 and B3, C2 andC3, D2 and D3, E2 and E3, F2 and F3, H2 and H3, and G2 and G3),determining an average of center point distances S in each group, andaveraging them over the selected four groups. The results are shown inTable 2.

TABLE 2 Distance Center Point Position of Cell Wall from Center DistanceS D⁽²⁾ Positional Relation No. Intersections⁽¹⁾ Axis (mm) (μm) (μm) ofC_(R) to C_(W) Example 1 Center Portion (A) 0.9 12.9 0  — IntermediatePortion (I) 67 25.3 — Outer Peripheral in Direction of 45° Wall Side⁽³⁾Intermediate Portion in 67 34.5 — Outer Peripheral Direction of 45° WallSide⁽³⁾ Intermediate Portion (G) 67 21.7 — Outer Peripheral in Directionof 90° Wall Side⁽³⁾ Intermediate Portion in 67 26.2 — Outer PeripheralDirection of 90° Wall Side⁽³⁾ Average in Intermediate 67 26.9 14.0 —Portion Outer Peripheral Portion 133 44.7 — Outer Peripheral (E) inDirection of 45° Wall Side⁽³⁾ Outer Peripheral Portion 133 41.5 — OuterPeripheral (F) in Direction of 45° Wall Side⁽³⁾ Outer Peripheral Portion133 38.7 — Outer Peripheral (B) in Direction of 90° Wall Side⁽³⁾ OuterPeripheral Portion 133 39.2 — Outer Peripheral (C) in Direction of 90°Wall Side⁽³⁾ Average in Outer 133 41.0 28.1 — Peripheral Portion Radiusof Radius of Circumscribed Position of Cell Wall CircumscribedCircle/Cell No. Intersections⁽¹⁾ Circle (μm) Wall Thickness Example 1Center Portion (A) 268 0.89 Intermediate Portion (I) 269 0.90 inDirection of 45° Intermediate Portion in 265 0.88 Direction of 45°Intermediate Portion (G) 266 0.89 in Direction of 90° IntermediatePortion in 263 0.88 Direction of 90° Outer Peripheral Portion 266 0.89(E) in Direction of 45° Outer Peripheral Portion 263 0.88 (F) inDirection of 45° Outer Peripheral Portion 265 0.88 (B) in Direction of90° Outer Peripheral Portion 269 0.90 (C) in Direction of 90° DistanceCenter Point Position of Cell Wall from Center Distance S D⁽²⁾Positional Relation No. Intersections⁽¹⁾ Axis (mm) (μm) (μm) of C_(R) toC_(W) Example 2 Center Portion (A) 0.9 6.0 0  — Intermediate Portion (I)67 33.2 — Outer Peripheral in Direction of 45° Wall Side⁽³⁾ IntermediatePortion in 67 29.4 — Outer Peripheral Direction of 45° Wall Side⁽³⁾Intermediate Portion (G) 67 25.8 — Outer Peripheral in Direction of 90°Wall Side⁽³⁾ Intermediate Portion in 67 26.8 — Outer PeripheralDirection of 90° Wall Side⁽³⁾ Average in Intermediate 67 28.8 22.8 —Portion Outer Peripheral Portion 133 41.3 — Outer Peripheral (E) inDirection of 45° Wall Side⁽³⁾ Outer Peripheral Portion 133 36.0 — OuterPeripheral (F) in Direction of 45° Wall Side⁽³⁾ Outer Peripheral Portion133 37.4 — Outer Peripheral (B) in Direction of 90° Wall Side⁽³⁾ OuterPeripheral Portion 133 35.8 — Outer Peripheral (C) in Direction of 90°Wall Side⁽³⁾ Average in Outer 133 37.6 31.6 — Peripheral Portion Radiusof Radius of Circumscribed Position of Cell Wall CircumscribedCircle/Cell No. Intersections⁽¹⁾ Circle (μm) Wall Thickness Example 2Center Portion (A) 267 0.89 Intermediate Portion (I) 268 0.89 inDirection of 45° Intermediate Portion in 271 0.90 Direction of 45°Intermediate Portion (G) 270 0.90 in Direction of 90° IntermediatePortion in 266 0.89 Direction of 90° Outer Peripheral Portion (E) 2650.88 in Direction of 45° Outer Peripheral Portion (F) 271 0.90 inDirection of 45° Outer Peripheral Portion (B) 267 0.89 in Direction of90° Outer Peripheral Portion (C) 268 0.89 in Direction of 90° DistanceCenter Point Position of Cell Wall from Center Distance S D⁽²⁾Positional Relation No. Intersections⁽¹⁾ Axis (mm) (μm) (μm) of C_(R) toC_(W) Example 3 Center Portion (A) 0.9 7.6  0.0 — Intermediate Portion(I) 56 22.1 — Outer Peripheral in Direction of 45° Wall Side⁽³⁾Intermediate Portion 56 21.6 — Outer Peripheral in Direction of 45° WallSide⁽³⁾ Intermediate Portion (G) 56 16.4 — Outer Peripheral in Directionof 90° Wall Side⁽³⁾ Intermediate Portion 56 18.2 — Outer Peripheral inDirection of 90° Wall Side⁽³⁾ Average in Intermediate 56 19.6 12.0 —Portion Outer Peripheral Portion 110 28.8 — Outer Peripheral (E) inDirection of 45° Wall Side⁽³⁾ Outer Peripheral Portion 110 32.0 — OuterPeripheral (F) in Direction of 45° Wall Side⁽³⁾ Outer Peripheral Portion110 33.5 — Outer Peripheral (B) in Direction of 90° Wall Side⁽³⁾ OuterPeripheral Portion 110 29.9 — Outer Peripheral (C) in Direction of 90°Wall Side⁽³⁾ Average in Outer 110 31.1 23.5 — Peripheral Portion Radiusof Radius of Circumscribed Position of Cell Wall CircumscribedCircle/Cell Wall No. Intersections⁽¹⁾ Circle (μm) Thickness Example 3Center Portion (A) 275 0.92 Intermediate Portion (I) in 260 0.87Direction of 45° Intermediate Portion in 261 0.87 Direction of 45°Intermediate Portion (G) in 262 0.87 Direction of 90° IntermediatePortion in 267 0.89 Direction of 90° Outer Peripheral Portion (E) 2630.88 in Direction of 45° Outer Peripheral Portion (F) 270 0.90 inDirection of 45° Outer Peripheral Portion (B) 264 0.88 in Direction of90° Outer Peripheral Portion (C) 264 0.88 in Direction of 90° Note:⁽¹⁾The Symbols (A) to (C), (E) to (G) and (I) express the positions ofcell wall intersections in FIGS. 13a-13c, and those without symbols arenot shown. Note: ⁽²⁾D represents the difference determined bysubtracting the center point distance S in the center portion from anaverage of center point distances S in various portions. Note: ⁽³⁾Thecenter point C_(R) of the circumscribed circle was deviated from thecenter point C_(W) of the cell wall intersection toward the outerperipheral wall.

The ceramic honeycomb structures produced in Examples 1-3 were measuredwith respect to B-axis compressive failure strength. The B-axiscompressive failure strength was measured on a test piece 17 of 24.5 mmin diameter and 24.5 mm in length taken out of each ceramic honeycombstructure, according to “Test Method of Ceramic Monolithic Carriers forExhaust Gas-Cleaning Catalysts for Automobiles,” M505-87 of the Societyof Automobile Engineers of Japan, Inc. The test piece 17 was taken suchthat the x-axis or y-axis of the ceramic honeycomb structure was inalignment with a compression load direction, and that the outerperipheral portion (excluding the outer peripheral wall and theoutermost incomplete flow paths) was included, as shown in FIG. 14 . Themeasurement of B-axis compressive failure strength was conducted onthree test pieces 17 taken out of each ceramic honeycomb structure, andtheir average was used as the B-axis compressive failure strength. Theresults are shown in Table 5.

Comparative Example 1

A honeycomb-molding die 20 having circular recesses 24 a, 24 b, 24 c, 24d of 540 μm in diameter in four corners of a die member 26 at each slitintersection 23 of slits 21 (300 μm in width, and 1.58 mm in pitch) wasproduced, as a die for extrusion-molding a moldable ceramic material. Inthis die, a die center point distance Sd between the center point Ca ofthe inscribed circle 24 of the circular recesses 24 a, 24 b, 24 c, 24 dand the center point Cs of the slit intersection 23 was 18 μm in thecenter portion, 18 μm in the intermediate portion, and 17 μm in theouter peripheral portion. In the die used in Comparative Example 1, theouter periphery 25 had a diameter of 277 mm. The structure of this dieis shown in Table 3.

TABLE 3 Distance Center Point Die Position of Slit from Center DistanceSd D⁽²⁾ Positional Relation Structure Intersections⁽¹⁾ (mm) (μm) (μm) ofCa to Cs Com. Ex. 1 Center Portion of die (A) 0.9 18.0 0.0 —Intermediate Portion (I) 67 18.0 — Outer Periphery in Direction of 45°Side⁽³⁾ Intermediate Portion (J) 67 18.0 — Outer Periphery in Directionof 90° Side⁽³⁾ Average in Intermediate 67 18.0 0.0 — Portion OuterPeripheral Portion 133 17.0 — Outer Periphery (E) in Direction of 45°Side⁽³⁾ Outer Peripheral Portion 133 17.0 — Outer Periphery (D) inDirection of 90° Side⁽³⁾ Average in Outer 133 17.0 −1.0  — PeripheralPortion Note: ⁽¹⁾The Symbols (A), (D), (E), (I) and (J) express thepositions of slit intersections in FIG. 10a. Note: ⁽²⁾D represents thedifference determined by subtracting the center point distance Sd in thecenter portion of the die from an average of center point distances Sdin various portions. Note: ⁽³⁾The center point Ca of the inscribedcircle was deviated from the center point Cs of the slit intersectiontoward the outer periphery.

The cordierite-based ceramic honeycomb structure of Comparative Example1 having an outer diameter of 266.7 mm, a length of 304.8 mm, a cellwall thickness of 0.3 mm, a cell pitch of 1.58 mm, and a cell wallporosity of 61% was produced in the same manner as in Example 1 exceptfor using this die. At cell wall intersections in the center, outerperipheral and intermediate portions of the cordierite-based ceramichoneycomb structure of Comparative Example 1, the circumscribed circlesof circular portions of fan-shaped bulges were measured as in Example 1,to determine their radii, the center point distances S each between thecenter point C_(R) of the circumscribed circle and the center pointC_(W) of the cell wall intersection, and their positional relations. Theresults are shown in Table 4. With respect to the center point distanceS at cell wall intersections in the center, outer peripheral andintermediate portions of the cordierite-based ceramic honeycombstructure of Comparative Example 1, the center point distance Sc in thecenter portion of the ceramic honeycomb structure and the center pointdistance So in the outer peripheral portion did not meet Sc<So. TheB-axis compressive failure strength of the ceramic honeycomb structureof Comparative Example 1 measured by the same method as in Example 1 isshown in Table 5.

TABLE 4 Distance Center Point Position of Cell Wall from Center DistanceS D⁽²⁾ Positional Relation No. Intersections⁽¹⁾ Axis (mm) (μm) (μm) ofC_(R) to C_(W) Com. Ex. 1 Center Portion (A) 0.9 12.9 0  OuterPeripheral Wall Side⁽³⁾ Intermediate Portion (I) 67 12.9 — OuterPeripheral in Direction of 45° Wall Side⁽³⁾ Intermediate Portion in 6712.8 — Center Axis Direction of 45° Side⁽⁴⁾ Intermediate Portion (G) 6712.8 — Center Axis in Direction of 90° Side⁽⁴⁾ Intermediate Portion in67 12.9 — Outer Peripheral Direction of 90° Wall Side⁽³⁾ Average inIntermediate 67 12.9 0.0 — Portion Outer Peripheral Portion 133 12.4 —Center Axis (E) in Direction of 45° Side⁽⁴⁾ Outer Peripheral Portion 13313.2 — Outer Peripheral (F) in Direction of 45° Wall Side⁽³⁾ OuterPeripheral Portion 133 12.3 — Center Axis (B) in Direction of 90°Side⁽⁴⁾ Outer Peripheral Portion 133 13.4 — Outer Peripheral (C) inDirection of 90° Wall Side⁽³⁾ Average in Outer 133 12.8 −0.1  —Peripheral Portion Radius of Radius of Circumscribed Position of CellWall Circumscribed Circle/Cell No. Intersections⁽¹⁾ Circle (μm) WallThickness Com. Ex. 1 Center Portion (A) 265 0.88 Intermediate Portion(I) in 265 0.88 Direction of 45° Intermediate Portion in 265 0.88Direction of 45° Intermediate Portion (G) in 265 0.88 Direction of 90°Intermediate Portion in 265 0.88 Direction of 90° Outer PeripheralPortion (E) 266 0.89 in Direction of 45° Outer Peripheral Portion (F)266 0.89 in Direction of 45° Outer Peripheral Portion (B) 266 0.89 inDirection of 90° Outer Peripheral Portion (C) 266 0.89 in Direction of90° Note: ⁽¹⁾The Symbols (A) to (C), (E) to (G), and (I) express thepositions of cell wall intersections in FIGS. 13a-13c, and those withoutsymbols are not shown. Note: ⁽²⁾D represents the difference determinedby subtracting the center point distance S in the center portion from anaverage of center point distances S in various portions. Note: ⁽³⁾Thecenter point C_(R) of the circumscribed circle was deviated from thecenter point C_(W) of the cell wall intersection toward the outerperipheral wall. Note: ⁽⁴⁾The center point C_(R) of the circumscribedcircle was deviated from the center point C_(W) of the cell wallintersection toward the center axis.

TABLE 5 B-axis Compressive Failure No. Strength (MPa) Example 1 2.3Example 2 2.5 Example 3 2.1 Com. Ex. 1 1.7

As is clear from the above results, in the ceramic honeycomb structuresof Examples 1-3 according to the present invention having fan-shapedbulges projecting in a fan shape toward flow paths from cell wallintersections at which cell walls were crossing, the center pointdistance So (distance between the center point of the fan-shaped bulgeand the center point of the cell wall intersection) in the outerperipheral portion of the ceramic honeycomb structure was larger thanthe center point distance Sc in the center portion (Sc<So), and thecenter point distance Sh in the intermediate portion between the centerand outer peripheral portions of the ceramic honeycomb structure waslarger than the center point distance Sc in the center portion andsmaller than the center point distance So in the outer peripheralportion (Sc<Sh<So). With such cell wall structure, the ceramic honeycombstructure produced in Examples 1-3 had high strength. On the other hand,the ceramic honeycomb structure of Comparative Example 1 had poorerstrength than in the present invention.

What is claimed is:
 1. A ceramic honeycomb structure comprising largenumbers of flow paths longitudinally formed by cell walls arranged in alattice pattern in cross section, and an outer peripheral wall formedaround said flow paths; at least one fan-shaped bulge projecting in afan shape toward said flow paths from a cell wall intersection at whichsaid cell walls are crossing, in a cross section perpendicular to thelongitudinal direction; circumscribed circles of circular portions ofsaid fan-shaped bulges at all cell wall intersections having a constantradius; and when the distance between the center point of saidcircumscribed circle and the center point of said cell wall intersectionis defined as a center point distance S, a center point distance So inan outer peripheral portion of said ceramic honeycomb structure and acenter point distance Sc in a center portion of said ceramic honeycombstructure meeting Sc<So.
 2. The ceramic honeycomb structure according toclaim 1, wherein the center point of said circumscribed circle ispositioned on the side of the outer peripheral wall of said ceramichoneycomb structure with respect to the center point of said cell wallintersection, in an outer peripheral portion of said ceramic honeycombstructure.
 3. The ceramic honeycomb structure according to claim 1,wherein the center point distance Sc in the center portion of saidceramic honeycomb structure is 20 μm or less, and the difference betweensaid center point distance So in the outer peripheral portion and saidcenter point distance Sc in the center portion is 5-150 μm.
 4. Theceramic honeycomb structure according to claim 1, wherein a center pointdistance Sh of the cell wall intersection in an intermediate portionbetween the center and outer peripheral portions of said ceramichoneycomb structure meets Sc<Sh<So.
 5. The ceramic honeycomb structureaccording to claim 1, wherein the radius of said circumscribed circle is0.75-1.25 times the cell wall thickness of said ceramic honeycombstructure.